The present invention relates to an apparatus for reading and/or writing data markings of an optical recording medium having data markings arranged along a track and header markings arranged laterally offset with respect to the centre of this track.
An apparatus of this type is disclosed in EP-A2-0 801 382. In this case, an optical recording medium is read which has data markings arranged along a circular or spiral track and also header markings arranged at specific intervals in the track direction. The header markings are in this case arranged laterally offset with respect to the track, in particular offset by half a track width. The known apparatus may be regarded as having the disadvantage that the read-out unit for reading out the data markings is optimized for the detection of markings which are arranged centred with respect to the track. Therefore, reading errors occur during the read-out of the header markings, which are laterally offset, that is to say arranged off-centre. In the case of the known apparatus, a reduction of the reading errors is obtained by virtue of the fact that, in order to read out the header markings, a difference signal formed from photodetector signals is used instead of the summation signal that is customary for the read-out of the data markings. This solution may be regarded as having the disadvantage that the evaluation unit is also not optimally adapted to this difference signal. A second signal processing path is prescribed as an alternative. This has the disadvantage of increasing the outlay on circuitry.
The object of the present invention is to enable data and header markings to be read out with the smallest possible error rate.
This object is achieved by means of the features specified in claim 1. In this case, the apparatus has a signal conditioner for forming a conditioned signal from an electrical signal generated by a photodetector, a header identification unit for identifying whether data or header markings are being read, and a switch unit, which is driven by the header identification unit and serves for connecting an input of the signal conditioner to different input signal sources. According to the invention, the apparatus furthermore has a threshold value forming unit for forming an input signal for the signal conditioner and a storage unit with at least two storage locations. By means of the switch unit, the input of the storage unit can be connected to the output of the threshold value forming unit and the output of the storage unit can be connected to a threshold value input of the signal conditioner. This has the advantage that reading errors are reduced by virtue of optimum adaptation of the signal conditioner to the signal, which is dominated either by header markings or by data markings. A different threshold value is fed as input signal to the signal conditioner for the purpose of reading out header markings from that which is fed for the purpose of reading out data markings. A further advantage is that in the event of a change from header to data markings, as a result of the changeover of the switch unit, a stored threshold value, that is to say one that is well adapted to the type of markings that is presently to be read out, immediately passes to the threshold value input of the signal conditioner. This threshold value is then subsequently additionally optimized by the threshold value forming unit.
A development of the invention provides for the switch unit to have three switch positions. This has the advantage that different threshold values are possible for data markings, header markings offset to the left and header markings offset to the right. In this case, it is necessary to provide a correspondingly larger number of storage locations.
According to the invention, the threshold value forming unit and the storage unit are realized on a digital basis. This has the advantage that interfering influences caused by fluctuations in the component properties which are unavoidable in the case of analogue components do not occur. In contrast to an analogue store realized by means of capacitors, storage that is virtually unlimited in terms of time, without any change to the stored value, is possible.
According to the invention, in each case two storage locations are provided for each switch position. This has the advantage that the series-connected storage locations have stored both the present and the preceding value of the threshold value, the preceding value in each case being utilized as input signal for the signal conditioner. In the event of an error occurring, the preceding, but correct value is no longer overwritten by the present, but already incorrect value, rather the correct value is utilized as input signal for the signal conditioner until the error no longer occurs.
A development of the invention provides for a switch to be present for connecting the threshold value input of the signal conditioner to the output of the switch unit or to the output of the threshold value forming unit. This has the advantage that in the normal case, that is to say when no error occurs, and when no transition from header to data markings or the like occurs, the present threshold value is always supplied to the signal conditioner. Given the presence of interference or in the event of a transition from header to data markings, on the other hand, a changeover is made to the stored, preceding threshold value, which is better adapted in this case.
The invention""s method for the error-free read-out of data and header markings of a corresponding recording medium provides for a threshold value to be formed by means of which a conditioned signal is formed from a detector signal. The present threshold value is stored in a first store during the reading of a data area, while the present threshold value is stored in a second store during the reading of a header area. Given the presence of a transition, for example from data area to header area or vice versa or within different types of data and/or header areas, on the other hand, storage is prevented and the stored threshold value corresponding to the type of area to be newly read is used for the purpose of forming the conditioned signal. In the event of a transition from one area to another, the type to which the following area to be newly read corresponds is generally known or can be ascertained. The method according to the invention has the advantage that after a transition from data area to header area, a poorly adapted, that is to say incorrect threshold value which settles only relatively slowly to the correct order of magnitude is not worked with, rather a threshold value that is adapted to the type of area to be read next is begun with immediately. This increases the scanning reliability and reduces the error rate.
A development of the method provides for the prevention of new storage and the use of the value already stored to be maintained for a predetermined period of time after the identification of a transition. This has the advantage that the value which is highly suitable for the type of area to be newly read is compulsorily retained directly after the transition. The system settles during this time, with the result that the system is stable at the end of this period of time and the newly formed threshold value has reached the correct value. The method is adapted to the properties of the apparatus by prescription of the period of time. A longer or shorter period of time may be expedient depending on these properties. The period of time should generally be as short as possible in order to obtain regulated adaptation of the threshold value relatively rapidly, but on the other hand should be long enough to prevent any errors that may occur in the transient recovery phase from taking effect. Also within the scope of the invention is that said time ends when the newly formed threshold value reaches a stable value or a value within a certain range around the stored value.
The invention provides for the prevention of new storage and the use of the value already stored also to be effected when a defect occurs. This has the advantage that the restart of the read-out after the defect is begun with a good threshold value, the stored threshold value, which is not corrupted on account of the defect. An example of what may be regarded as a defect in this case is an influence on the signal which is caused by scratches, dirt or the like and renders the read-out impossible for a short time, or at the very least results in a high error rate. Even in the case where an error is present, it is also generally possible to establish the type of area to be read after the error. This can be detected accurately in the case of slight errors; in the case of more serious errors, it is assumed as an approximation that the type of area valid last is also present after the end of the error. It is also possible to predict the type of area from known features of the disk, e.g. distance between succeeding header areas, and actual values, e.g. time elapsed after last header area, actual scanning speed etc.
According to the invention, in the event of a plurality of different types of header areas or data areas being present, further stores are utilized for storing and reading out the threshold values for the corresponding types of header areas or data areas. This has the advantage that the method can also be adapted to optical recording media which have headers offset both to the left and to the right of the track. It can thus also be adapted to other types of data areas or header areas provided with other different properties.
The invention furthermore provides for each store to have at least two storage locations, in which case storage is effected in a first storage location and reading is effected from a second storage location. In this case, the content of the first storage location is transferred to the second storage location in a time delayed or deferred manner, referred to as manner staggered over time, for example during the next or a following clock cycle. In order to interrupt the storage, the transfer from the first to the second storage location is prevented, with the result that the value of the second storage location is not updated, that is to say is retained. This has the advantage that it is not necessary to rapidly identify the transition or to rapidly identify errors. An already incorrect value stored in the first storage location does not yet influence the signal conditioner, which operates using the value situated in the second storage location, this value still being correct. Only the transfer from the first to the second storage location has to be interrupted. The time thereby gained generally suffices for reliably identifying a transition or a defect, even in the case of very high clock rates.